(a) hemoglobin SS or hemoglobin SB Sβ0 thalassemia and meet one of the criteria below for having severe sickle cell disease (i) Previous central nervous system event lasting longer than 24 hours, plus objective imaging evidence of CNS vasculopathy, with or without residual neurologic findings (ii) Frequent (≥ 3 per year for 2 years) painful vaso-occlusive episodes (defined as episode lasting ≥ 4 hours and requiring hospitalization or outpatient treatment with parenteral opioids/opiates). Must have also (iii) Recurrent (≥ 3 in lifetime) acute chest syndrome events which have necessitated exchange transfusion or chronic transfusion therapy. (iv) Any combination of ≥ 3 acute chest syndrome episodes and vasoocclusive pain episodes (defined as above) yearly for 3 years. (v) Stage I or II sickle lung disease (see appendix 1) (vi) Pulmonary hypertension, measured by tricuspid regurgitant jet velocity (TRV) of greater than 2.5m/s (vii) Osteonecrosis involving multiple joints. (viii) Sickle Cell nephropathy with moderately severe renal insufficiency estimated GFR ≥30 ml/min, but ≤60 ml/min/1.73 m2 (Requires evaluation by a nephrologist). (b) Thalassemia major (c) Glanzmann thrombasthenia (d) Wiskott-Aldrich syndrome (e) Chronic-granulomatous disease (f) Severe congenital neutropenia (g) Leukocyte adhesion deficiency (h) Shwachman-Diamond syndrome (i) Diamond-Blackfan anemia (j) Fanconi anemia (k) Dyskeratosis-congenita (l) Chediak-Higashi syndrome (m) Acquired (immune; non-inherited, non-congenital) severe aplastic anemia (only patients whose best graft source is a mismatched related donor, unrelated marrow donor or cord blood unit) (n) Other inherited or congenital marrow failure syndromes complicated by severe aplastic anemia (o) Other inherited or congenital red blood cell disorders requiring monthly chronic transfusion therapy. (p) Other inherited or congenital platelet disorders resulting in at least three inpatient hospitalizations in the past two years for bleeding. (q) Other inherited or congenital granulocyte disorders resulting in at least three inpatient hospitalizations in the past two years for infection.